The use of model giant axons in the squid and crayfish has led to the formulation of the "Glial-Neuronal Protein Transfer Hypothesis". In addition, the giant axon has been a useful model for the study of: 1) biochemical bases of excitability, 2) Intra-axonal Protein Phosphorylation, and 3) The regulation of axonal growth by Ca ions-activated proteases. The mechanisms underlying axonal transport have been studied using rat neurosecretory (hypothalamic) cells and rat sciatic nerve. These studies emphasize the diverse roles of cytoskeletal proteins (e.g., actin and tubulin) in fast vs. slow axonal transport. A novel method, using covalent labeling probes, to study axonal transport has been developed.